A colloidal silica is a dispersion of silica fine particles in a medium such as water etc., and is used not only as a property-improving agent, a paint vehicle, an inorganic binder, in the fields of papers, fibers, steel, and the like, but also as a polishing material for electronic materials, such as semiconductor wafers, etc. Particularly when a colloidal silica is used as a polishing material, the silica particles are required to have higher purity and higher density.
Examples of known processes for producing a colloidal silica that can satisfy the above-mentioned requirements include a particle growth method comprising continuously adding an alkyl silicate hydrolysis solution to hot alkali water. In this particle growth method, an aqueous active silicic acid solution is added under an alkaline condition. Therefore, spherical, mono-dispersed silica particles with a dense structure are likely to be formed.
In recent years, subjecting spherical, mono-dispersed silica particles to deformation (i.e., into secondary particles having a complex structure) so as to adjust a contact resistance between a surface to be polished and the deformed silica particles used as a polishing material, and thereby further improve a polishing rate, has been considered.
Patent Literature 1 teaches, in a colloidal silica production method using an aqueous alkaline silicate solution as starting material, that the reduction in the pH to 5 to 6 at one point of the particle formation event may result in the formation of two-particle aggregates, three-particle aggregates, or even larger particle aggregates. However, since an alkaline silicate is used as starting material in this method, the purity would inevitably be reduced due to the residual alkali metal. Further, Patent Literature 1 nowhere teaches obtaining a colloidal silica containing a large number of silica particles having a branched structure and/or a bent structure.
Known methods for producing deformed silica particles include, as described in Patent Literature 2, a pH adjustment by adjusting the amount of alkali added; an addition of a salt; a temperature adjustment; and an adjustment of anion concentration, particle concentration, etc. In particular, with respect to the addition of a salt, Patent Literature 3 teaches a method for preparing an elongated colloidal silica with the addition of a calcium salt or a magnesium salt. However, if the shape of silica particles is controlled with the addition of a salt, metal impurities will be introduced therein. For this reason, a colloidal silica prepared by such a method is not suitable for use in a semiconductor manufacturing process, which requires a high grade of silica purity.
In the Stöber method using alkoxy silane, particles having a nodular shape are likely to be obtained. Patent Literature 4 discloses that a cocoon-shaped colloidal silica can be obtained by adjusting four factors, i.e., the addition rate of alkoxy silane, the content of ammonium ion, the amount of water added, and the reaction temperature. However, since this method does not allow particles to slowly grow, unlike the particle growth methods, a colloidal silica obtained by this method has problems in terms of the particle density and residual silanol groups. Further, in a Stöber method, the particle growth conditions, such as reaction temperature, moisture, ammonia concentration, and addition rate, must be rigorously controlled. This makes the maintenance of a stable quality difficult.
Patent Literature 5 teaches a method for producing a colloidal silica, comprising performing hydrolysis by adding tetraethoxysilane to an aqueous hydrochloric acid solution, and adding the resulting silicic acid monomer solution to an aqueous ethylenediamine solution having a pH of 11.1 over 2.5 hours so as to allow particles to grow. This method also produces only particles having a subglobose structure, similar to the above-described Stöber method. Moreover, chlorine ions will be incorporated into the produced colloidal silica, posing a problem in relation to anion contamination.
Likewise, Patent Literature 6 discloses a method for producing an elongated colloidal silica from an aqueous active silicic acid solution that is obtained by hydrolysis of ethyl silicate with an acid. This method also poses a problem in terms of anion contamination, due to the acid addition. Further, the particles have an elongated shape; or a distorted spherical structure, such as a cocoon shape, nodular shape, or the like. Therefore, a colloidal silica comprising a large number of silica particles that have a branched structure or a bent structure is not obtained in this method.
Patent Literature 7 discloses a technique comprising treating an aqueous alkali silicate solution with a cation exchange resin; adjusting the pH of the resulting silicate solution to 1.0 to 7.0 using an alkali such as KOH, NaOH, water-soluble amine, or the like, or an inorganic acid or an organic acid such as a hydrochloric acid, a sulfuric acid, a formic acid, or the like; heating the resulting product for aging to thereby advance high-polymerization of the silicic acid; adding a water-soluble amine thereto to adjust the pH thereof to 9 to 12.5 to prepare a seed liquid containing nodular particles; and subjecting the seed liquid to building up (particle growth). In this method as well, although ions are removed by a cation exchange resin or an anion exchange resin, metal ions or anions in an amount on the order of parts per million (ppm) remain. Therefore, this method cannot produce a colloidal silica with a purity high enough for use in a semiconductor manufacturing process. In addition, the steps of this method are problematic, because a cumbersome ion exchange operation needs to be repeated many times.
As described above, the hitherto disclosed production techniques cannot produce a dense, high-purity colloidal silica containing highly deformed silica particles with a branched structure and/or a bent structure, and small amounts of metal cations and acid-derived anions.